An accurate knowledge of the state-of-charge of a cell or battery is often needed. For example, it is of critical importance that the state-of-charge of the batteries of an electric-powered vehicle be precisely known.
The fuel gauge of an internal combusion engine-powered automobile is a very useful instrument. Such an instrument permits a driver to estimate the remaining range based upon the amount of fuel in the fuel tank. The state-of-charge of the batteries of an electric-powered vehicle is analogous to the amount of fuel in the fuel tank and representative available number of ampere-hours which can be delivered by the batteries under specified conditions.
If a gasoline-powered automobile runs out of fuel, the driver will obviously be inconvenienced. However, a small quantity of fuel can be added to the tank so that the automobile may be driven to the nearest gasoline station for quick refueling. A stranded electric-powered vehicle presents a much more serious problem. Battery recharging equipment is very difficult to transport; therefore it would be impracticable under most circumstances to give a recharge to a stranded electric-powered vehicle. The vehicle would most likely have to be towed to the nearest recharging facility for a time-consuming recharge.
State-of-charge coulometers for electric-powered vehicles are notoriously inaccurate. In order to avoid becoming stranded, drivers add a considerable safety margin to the estimated state-of-charge. Thus, the effective range of an already range-limited vehicle is further reduced.
The state-of-charge of a battery is a function of many variables. The most important variables are obviously the quantity of charge (ampere-hours) added during charging and the quantity of charge taken out during discharge. These two variables can be readily determined using prior art integrating ammeters which are otherwise known as coulometers.
Unfortunately, the total quantity of the charge added during charging cannot be recovered during discharge. Stated differently, under most typical operating conditions, the coulombic efficiency (as opposed to energy efficiency) of a battery is less than 100 percent. There are inefficiencies during both the charge and the discharge cycles. Charge efficiency, which is usually more significant than discharge efficiency, is a function of charge voltage, charge current, battery temperature, battery age and other variables. Discharge efficiency is a function primarily of the discharge current, although other variables have an influence. However, there are no substantial coulombic inefficiencies in a lead/acid battery at normal discharge rates.
Prior art coulometers do not effectively take into account the major sources of coulometric inefficiencies of batteries. Accordingly, such devices are unsatisfactory for use in electric-powered vehicles and similar applications. The coulometer disclosed herein overcomes these limitations of the prior art device and is capable of accurately measuring the state-of-charge of a battery. These and other advantages of the subject device will become apparent upon reading of the following disclosure in conjunction with the drawings.